FIX: Prevent AssetAdmin duplicate confirmation for file deletion (fixes silverstripe/silverstripe-framework#2639)

We just needed higher specificity than the selector here: https://github.com/silverstripe/silverstripe-framework/blob/3.1/javascript/GridField.js#L139.

Frustratingly, using higher specificity for one of the two selectors doesn’t work - both need to be more specific. So despite the fact that the `.AssetAdmin.cms-edit-form .Actions button.action.action-delete` selector is never reached, we still need it in there. Perhaps @hafriedlander can advise if there’s a way around this?

The delete button in the ‘EditForm’ view for a file comes close to matching the second selector, but the `.cms-edit-form` is lacking the `.AssetAdmin` class.